All new Apple Watch apps will have to run natively on the Watch as of June 1st. Apple announced the mandate to developers last night, indicating that it would reject Watch apps from the App Store if they didn't comply.
Will this make the ecosystem stronger?
Developers have been able to publish native Watch apps since September, when Apple launched watchOS 2. Prior to that, all Apple Watch apps actually ran on the iPhone that the Watch was connected to. The iPhone would do all processing and essentially beam over to the Watch everything that it's supposed to display. It meant saving a bit of Watch battery life, but it also meant much slower apps, since you had to wait for information to be sent back and forth and back and forth.
Native apps haven't exactly made a huge difference in the watchOS ecosystem just yet. It still seems to be broadly accepted that you just don't use Apple Watch apps — there's little useful there. But this mandate will help to ensure that future apps are, at the very least, speedier than the ones available at launch.